1. Field of the Invention
The present invention relates to a semiconductor laser device having a compressive strain quantum well active layer above a GaAs substrate.
2. Description of the Related Art
Fujimoto et al. (xe2x80x9cHigh Power InGaAs/AlGaAs laser diodes with decoupled confinement heterostructure,xe2x80x9d Proceedings of SPIE, Vol. 3628 (1999) pp. 38-45) discloses an internal striped structure semiconductor laser device which emits light in the 0.98 Mm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type AlxGa1xe2x88x92xAs lower cladding layer, an n-type GaAs optical waveguide layer, an InGaAs quantum well active layer, a p-type GaAs first upper optical waveguide layer, and an n-type AlyGa1xe2x88x92yAs current confinement layer are formed in this order. Next, a narrow-stripe groove is formed, by conventional photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Thereafter, over the above structure, a GaAs second optical waveguide layer, a p-type AlGaAs upper cladding layer, and a p-type GaAs contact layer are formed. Thus, an internal striped structure is formed, and the semiconductor laser device oscillates in a fundamental transverse mode.
In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in the fundamental transverse mode can be realized by the difference in the refractive index between the n-type AlGaAs current confinement layer and the p-type GaAs second optical waveguide layer. However, the above semiconductor laser device has a drawback that it is difficult to form a GaAs layer on another AlGaAs layer, since the AlGaAs layers are prone to oxidation. In addition, since the optical waveguide layers are made of GaAs, current leakage is likely to occur. Therefore, AlGaAs leak-current protection layers are provided on both sides of the active layer. Nevertheless, the leakage current is still great, and thus the threshold current is high.
On the other hand, in order to prevent degradation of characteristics of the semiconductor laser device due to oxidation of aluminum included in an exposed regrowth boundary, T. Fukunaga (the inventor of the present patent application) and M. Wada have proposed a semiconductor laser device and a method of producing the semiconductor laser device in a coassigned and copending U.S. Ser. No. 09/634,703, filed on Aug. 7, 2000 and entitled xe2x80x9cHIGH-POWER SEMICONDUCTOR LASER DEVICE HAVING CURRENT CONFINEMENT STRUCTURE AND INDEX-GUIDED STRUCTURE,xe2x80x9d corresponding to Japanese patent application No. 11(1999)-222169, which is disclosed in Japanese Unexamined Patent Publication No. 2001-053383. In the above semiconductor laser device, the optical waveguide layers are made of InGaAsP, which has a greater bandgap than GaAs and does not contain aluminum. In addition, the current confinement layer is made of InGaP. Thus, the semiconductor laser device has a structure in which aluminum is not exposed on the regrowth layer. However, even in this structure, the leakage current is still great, and therefore the threshold current is high, since the band offset between the conduction bands of the InGaAsP and InGaP layers is small.
An object of the present invention is to provide a reliable semiconductor laser device which includes an internal stripe groove and a regrown layer over an internal stripe groove, and has the following features:
(a) Aluminum, which is prone to oxidation, does not exist on a regrowth boundary.
(b) The leakage current is suppressed by an index-guided structure formed with high precision.
(c) The semiconductor laser device oscillates in a fundamental transverse mode when the stripe width is small.
(d) The semiconductor laser device produces low noise when the stripe width is great.
According to the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type formed above the GaAs substrate; a lower optical waveguide layer formed above the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 and formed above the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1; a first upper optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 and formed above the compressive strain quantum well active layer, where x2=(0.49xc2x10.01)y2, and 0xe2x89xa6x2xe2x89xa60.3; a first etching stop layer made of Inx9Ga1xe2x88x92x9P of a second conductive type and formed above the first upper optical waveguide layer, where 0xe2x89xa6x9xe2x89xa61; a second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where x1=(0.49xc2x10.01)y1 and 0xe2x89xa6x1xe2x89xa60.3; a current confinement layer made of In0.49Ga0.51P of the first conductive type and formed above the second etching stop layer so as to form a second portion of the stripe groove; a second upper optical waveguide layer made of A GaAs formed so as to cover the current confinement layer and the stripe groove; an upper cladding layer of the second conductive type, made of one of AlGaAs and Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 and formed over the second upper optical waveguide layer, where x4=(0.49xc2x10.01)y4, and 0.9xe2x89xa6y4xe2x89xa61; a contact layer of the second conductive type; a first electrode formed on an exposed surface of the GaAs substrate; and a second electrode formed on the contact layer. In the semiconductor laser device, the first and second upper optical waveguide layers have an approximately identical refractive index, the upper and lower cladding layers have an approximately identical refractive index, the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm, and each of the lower cladding layer, the lower optical waveguide layer, the first and second upper optical waveguide layers, the first and second etching stop layers, the current confinement layer, the upper cladding layer, and the contact layer has such a composition as to lattice-match with GaAs.
Preferably, the semiconductor laser device according to the present invention may also have one or a combination of the following additional features (i) and (ii).
(i) The semiconductor laser device according to the present invention may further include first and second tensile strain barrier layers both made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5 and respectively formed above and below the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and the absolute value of the sum of the first product and a second product of the strain of the first and second tensile strain barrier layers and the total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(ii) The second etching stop layer may be one of the first and second conductive types.
The strain xcex94a of the compressive strain quantum well active layer is defined as xcex94a=(caxe2x88x92cs)/cs, and the strain xcex94b of the first and second tensile strain barrier layers is defined as xcex94b=(cbxe2x88x92cs)/cs, where cs, ca and cb are the lattice constants of the GaAs substrate, the compressive strain quantum well active layer, and the first and second tensile strain barrier layers, respectively.
When a layer grown over the substrate has a lattice constant c, and the absolute value of the amount xcex94=(cxe2x88x92cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the (GaAs) substrate.
When the thickness of the compressive strain quantum well active layer is denoted by da, according to the present invention, the above first product of the compressive strain xcex94a and the thickness da of the compressive strain quantum well active layer satisfies the following inequalities,
0 less than xcex94axc3x97daxe2x89xa60.25 nm.
In addition, when the semiconductor laser device according to the present invention has the additional feature (i), the absolute value of the sum of the first product and the second product of the strain xcex94b of said first and second tensile strain barrier layers and the total thickness db of the first and second tensile strain barrier layers satisfies the following inequalities,
xe2x88x920.25 nmxe2x89xa6xcex94axc3x97da+xcex94bxc3x97dbxe2x89xa60.25 nm.
Further, in order to substantially equalize the refractive indexes of the first and second upper optical waveguide layers, it is preferable to determine the composition of AlGaAs so that the difference between the refractive indexes of the first and second upper optical waveguide layers does not exceed 0.5%.
The semiconductor laser device according to the present invention has the following advantages.
(a) Because of the above construction, the semiconductor laser device according to the present invention can oscillate in a fundamental transverse mode in a wide range from a low output power to a high output power.
Specifically, in the above semiconductor laser device, a stripe groove is formed in the In0.49Ga0.51P current confinement layer of the first conductive type, and the AlGaAs second upper optical waveguide layer is formed so as to cover the current confinement layer and the stripe groove, where the second upper optical waveguide layer has the refractive index approximately identical to the refractive index of the first upper optical waveguide layer. Therefore, it is possible to maintain a difference in the equivalent refractive index between a portion of the active region under the current injection window and another portion of the active region under the current confinement layer in the range from about 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923. Therefore, it is possible to achieve efficient light confinement, and realize an internal current confinement structure and a real 15 refractive index guided structure with high precision.
(b) Since it is possible to increase the band offset between the conduction bands of the first and second upper optical waveguide layers, the leakage current can be suppressed, and oscillation with low threshold current density can be realized.
(c) When the upper cladding layer is made of AlGaAs having such a composition that the upper cladding layer has an approximately identical refractive index to that of the lower cladding layer, the temperature dependency characteristic of the threshold current can be improved.
(d) In the semiconductor laser device according to the present invention, the Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 second etching stop layer is formed under the In0.49Ga0.51P current confinement layer, and the second conductive type Inx9Ga1xe2x88x92x9P first etching stop layer is formed under the Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 second etching stop layer. Therefore, when the current confinement layer is removed by etching with a hydrochloric acid etchant, the Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 second etching stop layer is not removed by etching with the hydrochloric acid etchant. Thus, the etching with the hydrochloric acid etchant can be accurately stopped at the upper surface of the Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 second etching stop layer.
In addition, when etching with a sulfuric acid etchant is used, only the second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 is etched off, and the Inx9Ga1xe2x88x92x9P first etching stop layer is not etched. Therefore, the etching with the sulfuric acid etchant can be accurately stopped at the upper surface of the Inx9Ga1xe2x88x92x9P first etching stop layer.
Further, even when a GaAs cap layer is formed on the current confinement layer, it is possible to concurrently remove the GaAs cap layer and a portion of the Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 second etching stop layer exposed at the bottom of the stripe groove after the stripe groove is formed.
Furthermore, it is possible to enhance the controllability of the width of the stripe groove in wet etching, and accurately form the index-guided structure and the internal current confinement structure.
(e) Since the current confinement layer is arranged inside the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced.
(f) Since the layers exposed at the boundary on which the second etching stop layer is formed do not contain aluminum, regrowth of the second etching stop layer on the boundary is easy. In addition, since crystal defects caused by oxidation of aluminum can be reduced, the degradation of the characteristics of the semiconductor laser device can be prevented.
(g) When the first and second tensile strain barrier layers both made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5 are respectively formed above and below the compressive strain quantum well active layer, various characteristics of the semiconductor laser device are improved (e.g., the threshold current is lowered), and reliability is increased.